Switch mode power supplies (SMPS), because of their high efficiency, are popular in powering electronic circuitry such as CPUs, DRAMs, and ASICs. In contrast to the power devices used in linear regulators, the power devices in the SMPS work as ON or OFF switching components. Thus, the conduction power loss is minimized. However, the switching actions of these power devices will typically cause undesirable high frequency oscillations due to at least one of the following reasons:    a. parasitic inductance in the circuit board layout and power device package;    b. parasitic capacitance inherent in the power switching devices; and    c. series resistance or inductance in the capacitors.
A SMPS typically consists of at least two switching devices, with the first device being actively controlled to regulate the output voltage. The first device is usually referred to as the “main switch.” The second device is usually referred to as the “rectifier” that can be passively or actively controlled. One commonly observed high frequency oscillation is at the switching node of either of the power devices. Because this oscillation frequency is much higher than the normal switching frequency, the normally provided filter network of inductors and capacitors cannot suppress this ringing effectively. Consequently, the output may see high frequency ripples, resulting in possible electromagnetic interference (EMI) issues. In the prior art, a passive RC “snubber” is usually employed to suppress the ringing problem. Although somewhat effective, the RC snubber significantly increases the power loss.
The SMPS is popular in powering ASICs, DRAMs, and other electronic devices because of its high efficiency. The selection of the SMPS topology requires consideration of the relationship of the input and output voltages. One example is the portable devices such as digital cameras that use a single-cell Li-Ion battery to power a 3.3V rail, where the battery voltage is about 4.2V after charging up and drops to about 2.7V before the camera ceases to function. Appropriate efficiency is also required during the entire range of operation to maximize the battery usage life despite its limited size and weight. Such applications require SMPSs that can operate efficiently and automatically with input voltages that are above, below, or equal to the output voltage.
A “buck” converter can only be used if an input voltage remains higher than the output voltage. On the other hand, a “boost” converter may only be used if the input voltage stays less than the output voltage at all times. The well-known “buck-boost” converter can operate automatically from input voltages above, below, or equal to the output voltage, but cannot maintain a high efficiency over a wide range of input voltages. The buck-boost converter has a reasonable efficiency only when the input voltage is close to the output voltage, but it has much less efficiency than a buck converter when the input voltage is above the output voltage and much less than a boost converter when input voltage is below the output voltage.